Reflective optics are finding increasing use in optical systems due to the transmission losses, chromatic aberrations, lower damage thresholds, and higher B-integral accompanying the use of refractive optics. For example, ultrafast lasers and their applications rely heavily on broadband mirrors, because group velocity dispersion in refractive optics leads to pulse broadening, spectral distortions, and possibly beam breakup. The simplest reflecting objective is the spherical mirror. However, the spherical mirror has undercorrected spherical aberration for distant objects. In addition, coma and astigmatism are present in a spherical mirror when the aperture stop is not at the center of curvature. Parabolic mirrors have several advantages over spherical mirrors for aberration control. As with all conic surfaces, a point object located at one focus can be imaged at the other focus without spherical aberration. In particular, with a parabolic mirror, a distant axial object can form a diffraction-limited image at focus. A disadvantage of most reflective optical systems is that there is generally a central obscuration that can affect the light throughput and image contrast (e.g., Cassegrain and Newtonian type telescopes in astronomy). Therefore, a parabolic mirror with an off-axis aperture may be desirable to keep the focus out of the entering beam.
Commercial lenses and spherical mirrors are typically ground into a spherical shape, which only approximates a parabola at their very center. This leads to aberrations that are inherently present in all spherical optics. However, because of their uniaxial character, aspheric parabolic surfaces are generally much more difficult and costly to fabricate than ordinary spherical surfaces. Aspheric mirrors can be fabricated using computer-controlled diamond turning and traditional polishing methods, but material selection is limited and manufacturing costs can be large.
Fabricating off-axis aspheric surfaces is even more difficult. Currently, off-axis parabolic mirrors are cut from a parent paraboloid at the desired off-axis angle. While this method is feasible for small optics (e.g., 1-3 inch diameter) it becomes increasingly difficult to make large-angle, large-diameter off-axis parabolas. Therefore, almost all large off-axis focusing mirrors have off-axis angles of less than 5 degrees. The fabrication of larger off-axis angles is extremely expensive, because they require very big parent parabolas.
Furthermore, an as-machined mirror is not deformable and, therefore, does not enable variability of the focal length. Such deformable mirrors are highly desirable in imaging, laser projection, and astronomical applications. For these applications, deformable mirrors offer the possibility of improving the flexibility and capabilities of the imaging system while reducing size, weight and potentially cost.
Several prior inventions have used mechanical devices to elastically deform a thin mirror to provide a variable focal length deformable mirror. U.S. Pat. No. 3,972,600 to Cobarg discloses a deformable mirror that uses fluid pressure to elastically deform an edge-supported metal foil to provide a spherical mirror surface. The focal length of the mirror can be easily adjusted by varying the differential pressure across the foil. However, there are risks of fluid leakage and contamination of the mirror surface due to the pressurizing fluid, and the deformation provides a non-parabolic reflecting surface that is not optimum for many applications. U.S. Pat. No. 4,043,644 to Humphrey discloses a deformable mirror that uses a four-point load to elastically deform a substantially spherical mirror plate. The mirror is warped for off-axis use along an axis to provide a substantially toric shape that is not rotationally symmetric and can only approximate a parabolic mirror. U.S. Pat. No. 6,467,915 to Bar et al. discloses a deformable mirror that uses an annular pusher to deform a round mirror plate. However, because the outer edge of the mirror plate is rigidly mounted to a housing in an axial holder, and not simply supported, a non-parabolic surface results.
Therefore, a need remains for a variable focal length deformable mirror that has a parabolic mirror shape. Furthermore, a deformable mirror that enables an off-axis aperture is needed to keep the focus out of the entering beam.